DRY'd up a callback spec in draggable.
Bind callback methods to the plugin instance.
Callbacks now receive a draggable's original position, through the metadata object.
Reorganized the way we decide whether to calculate the helper's start position, or to read it from the draggable element.
Draggables that have implicit positions (auto left & right, or auto top & bottom) AND have undergone transforms now start their drags in the correct place.